feat(tasks): add recur aliases and RRULE support for task repeats

[salmonumbrella]

Summary

• add --recur and --recur-rrule flags to gog tasks add as aliases to repeat materialization
• parse RRULE input with strict support for FREQ + optional INTERVAL and reject unsupported/duplicate keys
• extend repeat scheduling to honor RRULE INTERVAL
• keep existing --repeat flow intact and add explicit validation messaging that true server-side recurring metadata is not exposed by the public Tasks API
• add/expand tests for new flags, RRULE parsing, and validation edges
• document materialization behavior in README tasks examples

Testing

• go test ./internal/cmd -run 'TestParseRepeatRRule|TestTasksValidationErrors|TestTasksAddCmd_RecurRRuleIntervalCreatesMultiple|TestTasksAddCmd_RecurAliasCreatesMultiple' -count=1
• ./.tools/golangci-lint run --allow-parallel-runners
• make test

Refs #401